358 



Journal of Agricultural Research 



Vol. IV. No. 4 



Table V. — Comparative counts obtained with four different types of aeroscopes 

 [ Bacterial counts given per liter of air ] 



Rettger 

 aeroscope. 



3- 

 4- 

 5- 

 6. 



7- 

 8. 



9- 



lO. 



13- 

 14- 



15- 

 i6. 



17- 

 i8. 

 19. 

 20. 



23- 

 24. 



25- 

 26. 



27. 



Average for 2 1 tests . 

 Average for 2 7 tests . 



23 

 67 

 26 

 14 

 14 

 98 



73 



52 



38 



60 



171 



30 



39 



73 



27 



211 



116 



77 

 80 



^33 

 31 

 36 

 65 

 38 



156 

 41 



343 



90 

 79 



Modified 

 standard 



aero- 

 scope.6 



IIO 

 129 



155 



75 

 34 

 25 

 33 

 56 

 III 



94 

 87 

 72 

 189 

 126 

 26 

 46 

 32 

 33 

 95 

 50 



80 



o The ordinary inlet tube of this form of aeroscope (see fig. 2) was replaced by an inlet tube of the diameter 

 and length of the inlet tube on the Rettger aeroscope. 



b No cork or inlet tube was placed in the upper end of these aeroscopes. Precipitation of dust was pre- 

 vented by means of a shield placed above the opening. 



EFFECT OF USE OF LIQUID FILTERS ON COUNT 



Since one of the chief differences between the Rettger type of aeroscope 

 and the modified standard aeroscope is in the filter and since there are 

 several possible ways in which the use of a liquid filter might tend to 

 lower the count, further investigations were made. As both distilled 

 water and physiological salt solution had been used in making the tests 

 with the Rettger aeroscopes, series of analyses were made with both of 

 these in which the bacterial suspensions were plated at once and again 

 three hours later. The actual time during which these substances might 

 exert a deleterious influence upon the bacteria was about 30 minutes 

 longer in the case of the Rettger aeroscope than the times given, as it 

 was impossible to make the first plating until about that interval of time 

 had elapsed after first starting the air bubbling through the liquid. 



While these tests were being made with the Rettger aeroscope, dupli- 

 cate tests were run with the modified standard aeroscopes. The bacterial 

 suspensions were plated in this case immediately after being made and 



